VSE render black as transparent
I'm a bit confused here, did you want to composite the images using the VSE or the compositor? No need to do it twice, either section is capable of doing what you're asking.

If the background is completely black you don't need to change the alpha.
A common way of compositing images with dark backgrounds is to mix them via luma keying.
You can try to add alpha channels, but an easier way for this application would be to just add the clips.

In the VSE this can be done by selecting the clips above your base clip and setting the blend mode to add instead of cross.



In the node editor add a color->mix node and set the mode to add.
Hope that helps.